Method and apparatus for producing synthetic plastics products, and product produced thereby

ABSTRACT

A process for working up thermoplastic synthetic resin material, in particular unsorted thermoplastic synthetic resin waste material into an article having the working and processing properties of wood, and in which the material is subjected to a mixing operation in a housing including a screw member, and is fluidized, moulded, cooled in the mould, and removed from the mould. According to the invention, the synthetic resin material is supplied to an extruder without a screen member and without an extrusion nozzle, and kept therein until there is obtained a material that is fluid owing to temperature increase, and which material contains a gas when it leaves the extruder. The material is then passed into a mould having open ends, one of which is connected to the outlet of the extruder.

BRIEF DESCRIPTION OF THE ART

This invention relates to a process for working up thermoplasticsynthetic material in particular unsorted thermoplastic synthetic wastematerial, into an article having the working and processing propertiesof wood, in which the synthetic material is subjected to a mixingoperation in a housing including a screw member, and is fluidized,moulded, cooled in the mould and removed from it.

Such a process for working up synthetic waste material is known by thename of "Reverzer" process, introduced by the Japanese CompanyMitsubishi Petrochemical, and mentioned by Shiro Hamava in "JapansPlastic Age", Vol. 12, Feb. 1974, pp 43-52, in particular pp 46-48, andfurthermore described in a leaflet on the process, published by theJapanese company. According to this process, sorted or unsortedthermoplastic waste material of for example, polyethylene,polypropylene, polyvinyl chloride or mixtures thereof, is comminuted,and the particulate material is subsequently stored in a storage space.In it, the particulate material is dried to a certain extent, using theheat released during the comminution of the waste material. From thestorage space the particulate waste material is supplied to the"Reverzer"-unit, which in essence is a melting unit, comprising ahoppper, in which the material is softened, and a mixer comprising ascrew member, in which the material is also fluidized. The fluidmaterial is then injected under pressure into a closed mould, and thusmoulded. The moulds are then transported to a cooling space, and cooledtherein by being sprayed with water, the moulds are opened and themoulded product is removed. The resulting products may have the form ofpossibly tapered beams, frames, U-beams, etc. depending on the mouldused. In this prior process, a high pressure is built up in the mixingand fluidization stage of the apparatus used, under the influence ofwhich, as stated, the fluid material is injected into specialpressure-resistant mould.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forworking up thermoplastic, synthetic material into shaped articles, inwhich specially adapted, pressure-resistant apparatus is unnecessary,but commercially available, non-pressure-resistant apparatus and dies ormoulds can be used.

According to the present invention, there is provided a process forworking up thermoplastic synthetic material, in particular unsortedthermoplastic synthetic waste material into an article having theworking and processing properties of wood, in which the syntheticmaterial is subjected to a mixing operation in a housing comprising ascrew member, and is fluidized, moulded, cooled in the mould and removedfrom the mould, characterized in that the synthetic material is suppliedto an extruder without a screen member and without an extrusion nozzle,and kept therein until there is obtained a material that is fluid owingto an increase in temperature, and which material contains a gas when itleaves the extruder, and which is then passed into a mould that is openon opposite ends, one of which ends is connected to the outlet of theextruder.

The process according to the present invention can be carried out usingany known, commercially available screw extruder, which may be of theadiabatic or of the normal type, but from which the pressure-producingcomponents (normally leading to pressures of 100-450 kg/cm²), that is tosay, the screen pack or the exchanger and the extrusion nozzle, shouldbe removed. The screw-type extruder is thus in essence transformed intoa screw conveyer, approximately like a meat mincer. The mouldingoperation, which in the process of this invention is carried out in aseparate step, should be effected in an open mould. As the material tobe moulded is introduced without pressure, tubes of round or rectangularcross-section, of simple standard quality can be used for making, forexample, round or rectangular poles or beams. The mould-filling pressureis provided in the process of this invention by the moulding gasavailable in the fluid material to be moulded.

The moulding gas can be obtained by providing the particulate syntheticmaterial to be supplied to the extruder with a blowing agent, thethreshold temperature of which is exceeded at the temperature occurringin the extruder. Suitable blowing agents are, for example, solidgranular blowing agents, suitably having a threshold temperature of150°-270° C., for example, azodicarbonamide (200° C.)diphenylsulfon-3,3'-disulfonyl hydrazide (150° C.),β,β-oxy-bis(benzenesulfonyl semicarbazide) (210° C.).

The proportion of blowing agent can determined by experimentation, itbeing inter alia dependent on the form of the ultimate product andproperties of the thermoplastic material. Suitable proportions are fromabout 0.4-1.2%, calculated on the weight of the mixture of thermoplasticmaterial and blowing agent.

When a gas is used as the moulding gas, for example, nitrogen or air,this is supplied to the extruder after the thermoplastic syntheticmaterial has been fluidized.

The process of this invention can be applied to all currentthermoplastics for example, low- and high-density polyethylene(LD/HDPE), polypropylene (PP), polystyrene, polyvinyl chloride (PVC),ABS in pure of mixed form. Mixtures of good processability are, forexample, mixtures of approximately 40% polypropylene+60% HDPE, or 40%propylene+60% LDPE, or 30% polypropylene+40% HDPE+30% polystyrene.

If the process of this invention is applied to synthetic resin wastematerial, this material will generally not be in a pure state butcontain impurities. In the process according to the invention, however,it is not necessary for the waste material to be previously cleaned.Generally speaking, the thermoplastic synthetic resin material to beprocessed may contain a certain proportion, for example, preferably nothigher than 20-25%, non-plastics materials, for example ground paper,card-board, textile fibres, sawdust, cheap filling agents, conventionalin the rubber and plastics processing industries (e.g. chalk, talc) andalso, for example, a pigment.

Depending on the type of screw extruder used, the material to beprocessed may be supplied in particulate form or not.

If the material to be processed is waste material, this can be used inthe form of a densified waste material or of waste material in granulateform. Depending on the form in which the waste material is supplied tothe extruder, unforced supply is possible if the material has asufficient bulk density, or forced supply if the waste material has avery low bulk density. Forcing can be effected, for example, by means ofa densifying screw.

After the moulding of the fluid material in the mould connected to theoutlet of the extruder, the mould is cooled, for example, by beingintroduced into a water bath, which is preferred, or by simply exposingthe mould to the air, allowing for a sufficiently long period ofcooling, so that the hardened and shrunk product can be removed from themould. It is recommendable for the product to be then further cooled inthe air, for which purpose it is laid on a flat substrate.

The process according to the present invention has many advantages,which can summarized as follows:

The process according to the invention is not critical. Using existingcontrol facilities of normal, commercially available screw extruders,the entire process can be easily controlled. As a screw extruder of theadiabatic type has smaller dimensions, this type is preferred asinvolving lower costs.

As no screen packet is used in the screw extruder, there is hardly if atall, any risk of clogging of the apparatus, so that the thermoplasticsynthetic resin material, in particular synthetic resin waste material,to be processed need not be previously washed of purified. The processaccording to the invention is also little critical with regard to thecomposition of the starting material, even to the extent that mixturesof unsorted, different thermoplastic synthetic resin materials turn outto be properly processable.

As there is no pressure build-up in the apparatus in which the processaccording to the invention is performed, the apparatus need not satisfyany special demands, for example, as regards the screw geometry. Also,the moulds need not satisfy any special requirements with regard topressure resistance, so that moulds of standard commercial quality maybe used.

The moulding operation does not require any calibrating devices.Moulding is achieved exclusively under the influence of expansion of thefluid material in the mould, effected by the gas made available therein.

Cooling of the material within the mould need only be continued untilthe product can be removed from the mould.

As the process is not critical, it can also be performed by means of atransportable apparatus mounted, for example on a motorlorry. Theadvantage is that the apparatus can always be installed in places wherelarge quantities of synthetic resin waste material become available.

The process according to the present invention even allows the use ofscrew extruders declared unfit for normal use.

The surface of the shaped product produced by the process according tothe invention has a specific structure rather unlike the surface of asynthetic material. The cause is that the foamed mass in the mould issubjected to the effect of shearing forces at the surface as a result oflocal solidifying processes while the non-solidified material flowsfurther owing to expansion.

The final product can be worked and processed as wood without anyfurther treatment. Its specific gravity is 0.7 kg/cm³ or higher. Thematerial is chemically resistant, its strenth is adjustable by means offillers, and the material is generally comparable to hard woods, such asazobe in the wet state. The process according to the invention can ofcourse be applied to thermoplastic synthetic resin material not yet usedbefore, but the greatest advantages of the process can be achieved usingthermoplastic synthetic resin waste material.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawings, which by way of example illustrate oneembodiment of apparatus suitable for the process according to thepresent invention.

FIG. 1 shows a diagrammatic side-elevational view of a processing andmoulding apparatus; and

FIG. 2 shows a cross-sectional view of the moulding section of theapparatus, taken on the line II--II of FIG. 1.

DETAILED DESCRIPTION

In FIG. 1, reference numeral 1 designates a hopper which when mixtureshaving a low bulk density are used is provided with a supply member 16for forced supply. Hopper 1 is equipped with an automatic dosagingdevice 2 for the supply of a blowing agent, either in the powdered form,or in "masterbatch" form. The blowing agent containing mixture is moltenin a screw extruder 3. Extruder 3 has no screen packet and no extruderhead. It may quite suitably be a short adiabatic extruder. Disposed inthe vicinity of the outlet of extruder 3 is a rotating mould holder 4comprising two disc-shaped supports 17, with which moulds 8-15 areconnected. Mould holder 4 is adjustable along a guide rod 18, so thatshorter or longer moulds 8-15 can be set up, and is designed toaccommodate moulds of different forms (round, square or flat). Moulds8-15 are forced against extruder 3 by means of a pneumatic or hydraulicpress 6 as they are being filled, but so that the end of the mouldfacing press 6 remains open. When mould 8 has been filled the pressurefrom press 6 is released, and the operation of extruder 3 is stopped. Asmould 8 is filled, the final product designated by 19 is removed frommould 19. Mould holder 4 is rotated, so that the empty mould 9 is movedinto registry with the outlet of extruder 3, and mould 8 is moved oneposition further. Mould 15 is dipped into a container 7, filled withwater. The extruder is then re-started. 20 is a handle for rotation ofholder 4.

The entire process of emptying a mould, stopping the extruder, removingpressure from the press, rotation, re-applying pressure to the press,can be effected automatically.

An important use of poles made the process according to the inventionlies in electricified fencing of fields where cattle is kept, oneadvantage being that the wires can be passed from pole to pole withoutthe use of insulators.

The process according to the invention is illustrated in and by thefollowing examples. In all the examples, use is made of an apparatus asshown in the accompanying drawings with a screw extruder of theadiabatic type.

EXAMPLE I

The extruder was fed with a mixture containing

30% ground LDPE bags scrap

30% ground HDPE rope scrap

39% ground polypropylene yarn waste

0.6% azodicarbonamide as a blowing agent.

From this mixture poles were made having a diameter of 150 mm and alength of 4000 mm. The specific gravity was 0.82.

EXAMPLE II

The extruder was fed with a mixture of:

15% ground waste paper

25% ground scrap from LDPE bags

39% ground LDPE packing nets

20% ground rope ends of polypropylene

1.0% azodicarbonamide as a blowing agent

From this mixture poles were made of 52 mm×52 mm×1500 mm. The specificgravity was 0.7

EXAMPLE III

The extruder was fed with a mixture of:

10% ground polystyrene coffee cups

40% ground polypropylene margarine tubs

10% saw dust

30% ground HDPE rope waste

0.8% azodicarbonamide as a blowing agent

From this mixture, poles were made of 52 mm×52 mm×1500 mm. The specificgravity was 0.75.

EXAMPLE IV

The extruder was fed with a mixture of:

50% mixed granulated sweepings from the floors of an extrusiondepartment. The sweepings consisted mainly of HDPE;

40% ground polypropylene rope waste from used and severely fauledhawsers;

10% ground sisal scrap;

0.5% azodicarbonamide as the blowing agent.

From this mixture, poles were made having a diameter of 100 mm and alength of 3000 mm. The specific gravity was 0.65.

I claim:
 1. A method of molding articles from thermoplastic syntheticmaterial, which comprises the steps of(a) simultaneously introducinginto an extruder apparatus having no screen member and no extrusionnozzle a quantity of a blowing agent and a mass of material at least 75%of which comprises a thermoplastic synthetic resin material; (b) heatingand mixing the components in the extruder apparatus to cause thesynthetic resin material to be in a fluid condition at atmosphericpressure; (c) introducing the mixture at atmospheric pressure into oneend of an open ended mold; and (d) cooling and removing the mixture fromthe mold.
 2. A method as defined in claim 1, wherein the blowing agenthas a threshold temperature that is less than the temperature at whichthe synthetic material becomes plastic.
 3. A method as defined in claim1, wherein said blowing agent comprises a gas.
 4. A method as defined inclaim 2, wherein the threshold temperature of the blowing agent isbetween about 150° C. to about 270° C.
 5. An article produced by themethod of claim
 1. 6. Molding apparatus for forming articles from a massof a material at least 75% of which consists of a thermoplasticsynthetic material, comprising(a) a screw extruder having an inlet andan outlet, said screw extruder having no screen member and no extrusionnozzle; (b) means for simultaneously introducing into said screwextruder a quantity of a blowing agent and a mass of material at least75% of which comprises a thermoplastic synthetic resin material, saidscrew extruder being operable to heat and mix the introduced componentsto cause the synthetic resin material to be in a fluid condition atatmospheric pressure; (c) open-ended mold means for receiving at one endat atmospheric pressure the material supplied from the outlet of saidscrew extruder; and (d) means for cooling the material following themolding thereof in said open-ended mold means.